Source of alt attribute is different depending on type of slider that you're using, usually it's based on a title of a slide. Besides that, if you're using lazy-loading, initial markup will contain link elements (<a>) instead of img.

If you wish to modify markup of slider image, in General options section click on Edit Slide Markup and replace {{image_tag}} with desired structure, for example:

Thank you for your response. I'm using a theme which has this plugin in-built but I don't see an option to add my own markup. I only have option to select the images to run a slideshow. Is there any other alternative way that I can add the alt tag for the images on the slider?